For seat belt apparatuses to be mounted in vehicles such as cars, various seat belt retractors including a pretensioner have been developed. In an initial stage of an emergency, such as a vehicle collision, when a deceleration higher than the normal deceleration is applied to the vehicle, a pretensioner rotates a spool of a seat belt retractor in a seat-belt retracting direction by using reaction gas generated by a gas generator, thereby winding a seat belt around the spool. This quickly removes slack from the seat belt and applies tension to the seat belt so as to increase the occupant restraint force.
As an example of a pretensioner of the related art, there is known a pretensioner in which a plurality of balls serving as force transmission members and a piston are stored in a pipe. The piston pushes these balls by receiving gas pressure of reaction gas generated by a gas generator in the event of an emergency, and thereby moves the balls along the interior of the pipe so as to press a plurality of pressed portions of a ring gear. When the ring gear is rotated by the press of the balls, a spool rotates in a seat-belt retracting direction to retract the seat belt (for example, see PTL 1).
In the pretensioner described in PTL 1, the pipe is provided with a gas release hole that connects the interior and exterior of the pipe so as to prevent the gas pressure in the pipe between the gas generator and the piston from excessively increasing.
As another example of a pretensioner of the related art, there is also known a pretensioner in which a rack and a piston serving as force transmission members are provided in a cylinder hole. The piston pushes the rack by receiving gas pressure of reaction gas generated by a gas generator in the event of an emergency. The movement of the rack causes rack teeth to rotate a pinion, and the rotation of the pinion rotates a spool in the seat-belt retracting direction so as to retract the seat belt (for example, see PTL 2).
In the pretensioner described in PTL 2, the piston and the rack respectively have exhaust holes that allow a gas generator side of the piston and a rack teeth side of the rack to communicate with each other.
In addition, in the seat belt retractor, when withdrawal of the seat belt is locked in the above-described event of the emergency, a great tension acts on the seat belt because of the inertia of the occupant. As a reaction force of the tension of the seat belt, a great force is applied from the seat belt to the occupant. Accordingly, there is also known a seat belt retractor including an energy absorption (EA) mechanism for limiting (suppressing) the force from the seat belt to the occupant (for example, see PTL 1 and PTL 2). As an EA member of the EA mechanism, for example, a torsion bar is often used. When the seat belt is almost withdrawn by the inertia of the occupant, the torsion bar is subjected to torsional deformation, absorbs the kinetic energy of the occupant, and limits the force applied from the seat belt to the occupant.
The EA mechanism starts subsequently to the completion of the seat-belt retracting operation of the pretensioner. When the EA mechanism performs the EA operation, the spool almost rotates in the seat-belt withdrawing direction, and therefore, the above-described force transmission members and the piston move in a direction opposite the moving direction during the operation of the pretensioner. Thus, the pressure in the pipe or the cylinder hole between the gas generator and the piston increases, and the piston receives resistance due to the increased gas pressure during movement.
Accordingly, in the seat belt retractor described in PTL 1, gas in the pipe between the gas generator and the piston flows outside through the gas release hole during the EA operation, so that the above-described resistance decreases. Further, in the seat belt retractor described in PTL 2, gas in the cylinder hole between the gas generator and the piston flows toward the rack teeth of the rack through the exhaust hole during the EA operation, so that the above-described resistance decreases.